text:
Turbofan engine 2 spool - Turbine drives the compressor or Starter drives the compressor
I have read somewhere regarding turbofan engine 2 spool that says

"LP or HP turbine will drive the LP or HP compressor"
"During engine starting, the starter will drive the HP compressor via the gearbox"

So I'm confused on whether the turbine or starter drives the compressor. Is it only during starting the starter will drive the HP compressor and after that turbine will take over?
Thanks

@ymb1 - OP: "I'm confused on whether the turbine or starter drives the compressor", so isn't your proposal for a duplicate far fetched? The OP is asking about the mechanical link between the accessory box and the N2 spool, as answered by John K.

The starter drives the entire HP spool (HP compressor, interconnecting shaft, and HP turbine).  The gearbox in question is the accessory gearbox that runs the various pumps and is driven off the HP spool shaft, which by happy coincidence is on the outside of the LP shaft.  The starter is mounted on that gearbox and is able to back drive the HP spool through the gear train for engine starting.
The LP spool, which is the fan and LP turbine with its shaft running down the center of the HP spool, is just going along for the ride during the start.

Question 1
"The LP spool, which is the fan and LP turbine with its shaft running down the center of the HP spool, is just going along for the ride during the start. " - does it mean that the LP shaft is connected to the HP shaft? So once HP shaft turns, LP shaft will turns too?
Question 2
So once starter cuts off, everything will run on its own?
Thanks

The LP and HP shafts have no mechanical connection. But the air displaced by the LP compressor will start turning the HP compressor.

That's what I meant by "going along for the ride".  The LP shaft has a set of bearings that suspend it within the HP shaft and it is free to spin independently.  Replace the fan at the front end with a gearbox and propeller, and you have a free-turbine turboprop.  In a TP, the HP  rotor (comp/shaft/turbine) and the burner can between them is called the "gas generator".  On a turbofan it's called the "core".  The  starter system winds up the core to start the engine.  The fan only starts to turn when the airflow through the engine starts spinning its turbine.

So during starting, the HP spool will be turn by the starter then the air displaced by the HP compressor will start turning the LP spool?

To the extent that there is enough air moving past the LP turbine blades to make the disc start to spin, yes.  Depending on the engine, you might see the fan start to rotate slowly on the flow past the LP turbine being created by the starter alone, but not really start to move until light off.

So starter is the one who is doing all the job "start turning", then why in some books i read they say turbine helps to turn the conpressor?

The turbine may start to produce a bit of torque to offset the load of the compressor, to the extent that the flow through the cold engine being generated by the compressor applies a bit of torque at the turbine, but this is fairly minor.  Doesn't start to generate any significant torque until there is flame in the burner can.  Even after lightup, with hot gases now going through the turbine, the flow has to increase to a certain level before the turbine can produce enough torque to support the compressor's load without the help of the starter.


text:
Why does the UK CAA restart its page numbering with every new chapter of their CAP publications?
Every CAA Publication (CAP) seems to restart its page numbering when starting a new chapter. Does anyone know why they have decided to do this?
For example, the CAP413. Instead of a logical page number ordering throughout the document, you have to reference it as chapter 4, pg 4 or chapter 5, pg 2.
It makes it very difficult to reference information to others. It also makes navigating as a PDF challenging - many of the CAPs are solely in PDF format. (Often you can only view one page at a time, and it is so easy to get lost in a document which restarts its page ordering).
Is there a reason why the CAA has done this? What would be the formal or best way of referencing a CAP page?

That's an artefact of a different era. Before computers were used to create documentation, updating large documents was difficult and expensive. People tried to reduce the workload by using chapter-page numbering. When you have to add pages to a book, chapter-page numbering allows you to replace one chapter instead of having to reprint the whole book.
Some organizations even supplied book updates as individual pages with a 'list of effective pages' so you could swap them out in your copy.
Nowadays there's no reason to keep doing this. Rebuilding a document (generate the TOC and crossreferences) is cheap, printing is cheap. You already noticed the big disadvantages of chapter-page numbering to the user.
Why they're still doing this? Inertia, probably.
How to reference them? Page 5-2 works, and if you want to be comprehensive, page 5-2 (PDF page 124).
(sorry, no references. Just lots of experience building large documents)

I think the UK government invented inertia.

Yep the CAA are perhaps the most backward government department technologically. I have it on fairly good authority that their very delayed "new computer system" is to replace an old system using Win95 terminals.

@GdD Unfortunately, Newton didn't take over the Royal Mint until a decade after inventing inertia.

There's still NT in places @Jamiec in the UK gov. I am not kidding. A few years ago in a a non-government client I found OS2! I thought it must have been a mis-detection, but no, it was in charge of a critical industrial control system!

@GdD: If you think that's amusing, you'll love this: In October, the United States Air Force announced that it had worked out how to launch a nuclear missile(!) without using 8-inch floppy disks.

If some chapters will change much more often than others, but the chapters that change infrequently will often be cited, chapter-page numbering offers advantages to users as well as publishers.  If I have a citation for page 5-137 of the 49th edition, and I have the 53rd edition, but chapter 5 hasn't changed, the relevant text would be found on page 5-11 of my edition even if chapter 4 had gained 19 pages.

@Kevin in their defence, if there’s one system I really don’t want to be screwed up after an “improvement”, the nuclear launch system is probably it. Relying on an old system has downsides, but the upside is: it’s worked for 50 years, and it’s likely to keep working for another 20...

source: Stack Exchange

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sort: votes


text:
Is it necessary to strengthen the airframe of an aircraft that is used for sustained low level flight?
I was looking at the image of the A 10 and saw that its air frame looks far more stronger than the air frame of the F 15 or the F-16.

I realized that the A 10 is used for undertaking low-level flights for deep interdiction. Apart from the A 10, the French Rafale fighter is also used for low level flights.

So is it necessary to strengthen the air frame of such aircraft or are they built from scratch with a material that is different from the ones used to design aircraft like F 15 and F 16?
I've heard of designers strengthening high stress areas of air frames with fiberglass,Coffee filters etc. However, increased weight will translate into higher wing loading which means more landing and flying stresses.
EDIT:
Sustained low level flights are associated with more structural failures in fighter aircraft. This is why a number of Air Force the world over have mission specific aircraft. Basically what I need to know is whether the airframe  of a fighter aircraft that is generally used for low level flight(A 10, Rafale, Su 34) any different from the airframe of other fighters like F 15, F 16 or Typhoon?
To be sure, I got the answer from the posts below, but just wanted to clarify as this question is on hold.

I'm not sure how you're quantifying the "strength" of an air frame - you may want to edit your question to expand on that concept. Each air frame is designed to withstand the loads expected to be put on it during its flight regime. One thing I do know for certain - significant portions of the A10 are armor plated to protect the plane from small arms fire since it does fly rather low.

"coffee filters"? I would like to see a reference for that one...

F-15 and F-16 are also used for undertaking low-level flights as is any other fighter, because flying under radar is an essential tactics.

@voretaq7 -Coffee filters make a great substitute for fiberglass to strengthen areas. Once saturated with epoxy the resulting matte is light and strong.

@CSinha I don't doubt that theycould be used, but I've never heard of this being used in a production aircraft, which is what I'd like to see a citation for (I'm fairly sure AC 43-13 doesn't mention the use of coffee filters in composite construction - they expect you to cut fiberglass/kevlar/carbon fiber weave to the appropriate size & lay it up in the usual way).

The A-10 in that sturdy because is designed to fly under enemy fire

Since the question you're ostensibly asking is "Do you need to strengthen the airframes of aircraft used for sustained low-level flight" let's leave aside the military aircraft in your question and just look at "sustained low-level flight" in general.
But first let's bear in mind two concepts:
The first concept is that all aircraft (indeed, all structures) have a finite fatigue life: through normal use, vibration, operating stresses from takeoffs/landings/maneuvers, etc. an aircraft accumulates a certain amount of fatigue, and at some point that fatigue will lead to failures (bending or cracking) in the aircraft's structures.
Aircraft are engineered to withstand a certain amount of fatigue (the "fatigue life" of the airframe) based on a number of factors, and when the fatigue life limit is reached the airframe is either retired (scrapped) or rebuilt/reinforced to extend the fatigue life.
The second concept is something in fluid dynamics called a boundary layer - the region of a fluid in contact with (and most influenced by) some surface.
For purposes of flight we can consider air (the atmosphere) to be a fluid in contact with the Earth's surface. This means down close to the Earth's surface there's a boundary layer with associated meteorological phenomena.
The first article in this NOAA piece talks about some of them, but for purposes of this discussion let's just reduce it to "It's really bumpy and gusty in the boundary layer. Once you climb above the boundary layer the air is smoother."

Bearing these two concepts in mind, an aircraft that spends most of its time operating down load (near the surface of the Earth) is also spending most of its time inside the boundary layer, where it gets bounced around and shaken up more by the turbulence in the boundary layer, and thus accumulates fatigue at an accelerated rate.
Because of this aircraft that spend a lot of time in the boundary layer are usually "overbuilt" to handle a greater degree of fatigue/stress, or have reduced service/fatigue life limits to ensure the aircraft is inspected and repaired/retired before fatigue can lead to a failure.
The accelerated fatigue from spending a lot of time in the boundary layer can also be dealt with by inspections rather than engineering changes. Piper Service Bulletin 886 highlights a particularly dramatic case:

On March 30, 1987, a PA-28 engaged in pipeline patrol operations suffered an in-flight wing separation resulting in a fatal accident. Investigation revealed the wing failure was due to propagation of a fatigue crack, which originated in the wing lower main spar cap.

Piper then goes on to describe "Normal" and "Severe" usage of the PA-28 family of aircraft, broadly speaking Normal is flying "normally" (Take off, climb to an altitude generally above the boundary layer, fly to your destination, and land), while Severe usage involves spending a lot of time in the boundary layer ("below 1000 feet AGL"). They then recommend inspection intervals based on the type of usage the aircraft is seeing (around 60,000 hours for "normal" usage, and roughly half that for "severe" usage due to the accelerated fatigue accumulation).

TL ; DR
Most aircraft that spend most of their time at low altitudes (close to the ground) are "overbuilt" structurally because there is more turbulence down low, so they accumulate fatigue at an accelerated rate.
Plenty of "normal" aircraft also spend a lot of their life at low altitudes, and this is usually handled by inspecting them for signs of fatigue damage more frequently than their higher-altitude counterparts.

I wonder if higher abrasion / corrosion are factors too at low level flying. It'd seem a lot more hot, humid, corrosive air and also the chance of dust and other debris scouring the fuselage.

@curious_cat That depends on the environment the plane is being operated in (one doing pipeline patrol in the middle of a desert would be less at risk for corrosion than one doing it in the tropics, though both would still get bounced around in boundary layer turbulence). Broadly speaking though environmental factors like moisture-induced corrosion would be considered separately from something like fatigue accumulation from low-level flight: The former accumulates by flight hours, while the latter accumulates on calendar time (& doesn't stop when you're parked on the ground).

The A-10 looks the way it does because it is a plane designed not just to fly low, but to fly low, slow, and fully-loaded over terrain containing a lot of people who want to shoot that aircraft down, because they know if they don't do it quickly, they're dead. Every aspect of the A-10's design is intended to facilitate the aircraft's use in such an environment and give it the highest chance of coming back.
The A-10 is actually not intended for deep interdiction; that mission is more often given to the F-15E, F-16, or the B-1B, depending on range, payload requirements and availability. The A-10, especially the newer C variant which has an onboard laser designator for smart bombs, can go bunker-busting behind enemy lines if it has to, but its maximum speed and service ceiling are too low for real penetration work.
The A-10 was built for a single purpose: close air support (or CAS). It was designed to take a lot of weapons into the sky over the front lines, supplementing and supporting friendly infantry and armor by taking out enemy armor and artillery. Most of those enemies would be able to shoot back, using anything from AK-47s and shoulder-fired Stingers to MPAT shells and mobile AAA/SAM launchers like the SA-13 or ZSU-23. To that end, it was built for a high payload capacity, good close-quarters maneuverability, long loiter times, extreme durability, and an on-board cannon capable of engaging armored targets (these last three elements being totally absent from Vietnam-era F-105 and F-4 ground attack aircraft, while Marine Hueys and Cobras had insufficient armor-piercing capability to engage the NVA's Soviet-supplied tanks).
These elements also make the aircraft good for other shorter-range ground missions, such as BAI (Battlefield Air Interdiction; destroying ground forces behind the front lines on their way to make trouble) and SEAD (Suppression of Enemy Air Defenses; this task is more often given to F-16s which can carry the HARM anti-radar missile, but if the A-10 can see a SAM launcher, it can kill it). While not designed for the task, the A-10 is more than a match for enemy helicopters, typically carrying a pair of Sidewinders under one wing (a 30MM burst from the GAU-8 will also do the trick).
It was also built on a budget; with the F-14 and F-15 air superiority programs costing over \$30 million a plane, getting the additional money for a dedicated small ground attack plane through Congress was difficult, so the winner of the AX program had to have a unit cost less than \$1 million. That meant that researching radically new wing planforms and fuselage design concepts just wasn't in the budget; both the A-10 and the YA-9 that lost to it were built very simply using proven design elements hearkening back to the first generation of jet fighters and even to WWII-era light bombers. The A-10A variant originally cost just \$450,000; modern A-10Cs, with glass cockpits and more advanced targeting systems for smart weapons, are still only \$11 million in more recent dollar figures. Compared to the F-15E's \$31 million unit cost, and the A-10's replacement the F-35 costing \$98 million each, the A-10C is still an unbelievable bargain.
As a result of all these design requirements, the A-10 has straight wings with a much thicker cross-section, generating more lift at lower speeds and a smaller target area than the F-15's tennis-court-sized planform. Those thick wings are also designed for 6g maneuvering with up to 16,000 pounds of ordnance hanging from them. An additional reason for so much lift and wing volume is that the aircraft is designed to fly with half a wing missing (in addition to an engine and half its tail), and has dual redundant hydraulic loops to all control surfaces plus a set of good old-fashioned cable controls should the main hydraulic or electrical system fail (the core components for both of these being housed in the "titanium tub" that protects the pilot and core avionics).
The design considerations for the F-15 were different. The requirements for the Eagle favored high speed and maneuverability (the MiG-25 and its purported capabilities had really scared the USAF top brass; it turned out to be fast, but really heavy, and so it wasn't the dogfighter NATO feared it would be when they saw its wings in recon pictures), a high ceiling (it's called the Eagle for a reason) and "not a pound for air to ground", focusing on BVR aerial engagements using Sidewinders and Sparrows (now AMRAAMs) with the additional ability to mix it up at visual range with Sidewinders and guns. In short, the USAF wanted the best pure air-to-air fighter money could buy, and for its day they spared no expense; the F-15C has a unit cost around \$30 million, figured primarily in late 1970s dollars (the equivalent of $133 million a plane today, not that much less than the Raptor program).
The F-16, and its runner-up in the LWF competition the YF-17 (which the Navy would develop into the F/A-18), had a slightly different purpose; to be the best dogfighter that $16 million would buy, that could also drop bombs. The idea was to supplement the ridiculously expensive F-15 with something the USAF could afford to lose a few of in establishing air superiority and suppressing ground-based air defenses. The program emphasized a small, nimble, affordable fighter capable of attacking any target the USAF would be assigned, air or ground.

The A-10 was built to be shot at, the F-16 to avoid being shot at. The A-10 is essentially a flying tank / gunship, it was (quite literally) designed around its massive gun.

First, I disagree that the A-10 airframe "...looks stronger than..." that of the F-16.  But that's a personal opinion.
I would like to emphasize the concept mentioned by others:  Each of these aircraft is designed and built to execute a role on (above?) the modern battlefield.  Almost by definition each is strong enough to fulfill that role.  Mistakes have been made (see the re-design and reinforcement of the C-5 cargo plane's wings as one example) but that is what happens when the designers push the envelope to produce a state of the art aircraft.
So the answer to your question is "yes, the planes are purpose built for their role."  Also true, however, is that the engineers are always learning from the successes and failures of engineers on other planes.  And incorporate the good ideas and good new applications of technology on the next plane to be designed and built.

"Strength" is not really something you can deduce from simply looking at an aircraft. That being said, in a manner of speaking the A-10 may be "Stronger" than its F-16 counterpart but they fill different roles so you are comparing apples to oranges.
The A-10 does have titanium armor plating around the cockpit which I don't think the F-16 has

"The single-seat [...A-10] cockpit is protected by all-round armor, with a titanium 'bathtub' structure."

There are also of course stores that must be taken in context like

The A-10's durability was shown on 7 April 2003 when Captain Kim Campbell, while flying over Baghdad during the 2003 invasion of Iraq, suffered extensive flak damage. Iraqi fire damaged an engine and crippled the hydraulic system, requiring the aircraft's stabilizer and flight controls to be operated via the 'manual reversion mode'. Despite this damage, Campbell flew the aircraft for nearly an hour and landed safely.

which of course can be turned into people thinking the A-10 is "Stronger".
And then there are pics like this that make it look like the plane can take quite a beating (which apparently it can)

Now lets look at some airframe specs
The A-10 load limits are

The maximum G-loads for the A-10 with a weight around 30,000 pound is +7.33/-3.0 G symmetrical and +5.80/-1 G asymmetric.

While the F-16 racks in at

F-16 was the first fighter aircraft purpose-built to pull 9-g maneuvers and can reach a maximum speed of over Mach 2

so in that regard you may be able to argue that the F-16 is stronger due to its maximum load limit.
For what its worth the exterior of a plane sheds little light on the airframe inside. Here is an example, this nice little single seat Mooney Looks pretty strong, but its made of plywood and fabric mainly.

'so in that regard you may be able to argue that the F-16 is stronger due to its maximum load limit'       But then for sustained low level flights the F 16 is not used unlike the Rafale and A 10. Is it because the airframe of the A 10/Rafale is stronger than the F 16

Again, I'll ask, how do you define "strength", @CSinha? Is an A380 or a C5-B "strong" because it can carry a lot of weight? Is a C130 "strong" because it can circle a hot LZ all night long firing .50-cal rounds into the surrounding area? Is an SR-71 "strong" because it can fly faster and higher than any other aircraft known to civilians? Each and every aircraft has its strengths - it is strong in the area(s) it was designed to be strong in.

Plywood and fabric can be pretty strong, for what it's worth. :)

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Is there any authoritative source where I can find a list of companies producing Sustainable Aviation Fuel in the U.S?
Is there any authoritative source (website, directories, etc.) where I can find a list of companies producing Sustainable Aviation Fuel (SAF) in the U.S?
Based on media reports it seems that there are close to 50 companies currently that are producing SAF in the U.S, however after extensive research I could find a list of 10 companies only.
I could find quite a few such companies operating in Europe. Some of them have operations in the US. However, couldn't find any source that states how many such companies operate in the U.S

Will appreciate any help in this regard.
Thanks

What media reports indicate 50 sources? Where did you find a list of 10?

@tedder42 Various magazines that focuses on green fuel, occassionally a few TV stations. They suggest that there are close to 50 such companies in the US, but does not mention all the companies. So I went through several of these magazines, websites and so far found the names of 10 companies.

There may be 50 companies researching sustainable aviation fuels, but I'd be surprised if there were even 10 producing them.

@RonBeyer There are more than 10 companies producing various types of SAF here in Europe itself. I have added an image to illustrate that. Highly unlikely that the corresponding figure for the US will be drastically low, especially given the recent focus on SAF.

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text:
Is flying a UAV via a VR controller (e.g. Oculus) legal in the US?
I've heard claims that flying a UAV using a Virtual Reality headset (for example, the oculus rift) is illegal per FAA regulations.
Is this true? I cannot find any definitive answer either way.

According to Model Aircraft Operations one of the requirements for flying a model aircraft is:

Keep the aircraft within visual line of sight at all times

If you are wearing a VR headset, then your visual line of sight stops at the headset screen.
Note that the above linked page lists the "visual line of sight" as a safety guideline. However, according to Section 336, an aircraft is not considered a "model aircraft" unless it is:

(2) flown within visual line of sight of the person operating
  the aircraft

None of the above restricts you from flying with a friend where one of you is wearing the VR headset and the other is piloting the aircraft.

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pressure wave propagation of shockwaves?
I can’t find a simple explanation of how the pressure waves propagate that leads to a shock over an aerofoil online.
How does this happen with upstream flow that is high speed but subsonic?

You should ask "please" at least seven time to get an answer

Isn't this the same as your last question https://aviation.stackexchange.com/q/101538/7532


text:
What type of visa is required to be allowed to earn a PPL in the US?
What are the visa types (H1b, L1b, H4, L2, J1, etc) accepted by TSA to learn flying for recreational purposes?
UPDATE 1
I found some further information from National association of flight instructors, According to them; Person on work visa can follow Part 61 course and get the PPL.
UPDATE 2
I have received the permission from the TSA to flow the Category 3 flight training. It took around 1 & 1/2 weeks to process and it will cost around 130 for the application and 99 for fingerprint process.
Based on this process, I understood that L1 Or L2 Or H1 Or H4 holders are eligible with the right set of document and no criminal record.

I got there on a F1 vor my PPL and Instrument rating

For a foreigner to come to USA to learn to fly, one would need an F-1 or M-1 visa.
This article in Flying magazine provides some information. You can get more information on from this school1. Also, you can look at this meta question: Can I ask about details of coming to the US to get a PPL?
Update:
It appears that you are already in the US on H-1b visa (it wasn't mentioned in the question), you need to go through the security clearances mentioned in Alien Flight Student Program (official version on TSA website).
AOPA has put together the related information (website). I also found about this flight school1 which has outlined the process.
If you search on Google about Alien Flight Student Program, you will find a ton of information.

1: Disclaimer I am not advertising any flight school.

Those visas are the primary student visa types, and would allow a foreigner to come to the U.S. specifically to attend a flight school (flight school is a "vocational training program" and so people coming specifically to get a pilot's license would probably get an M-1). Another possibility is an H-3, if you live in a country that has no aviation education facilities locally and you're not trying to become a commercial pilot. Immigrant visas leading to permanent residency would also qualify. You might also be able to train on your own time while in the U.S. on other business, such as an H1B.

KeithS, Do you have any reference for the statement of H1B holders can learn for the recreational purpose>?

@Vanji I've updated my answer as it appears you are already in the country.

@Vanji - http://www.askacfi.com/9242/are-non-us-citizens-eligible-for-private-license.htm. In short an H1B visa holder, like any, is subject to a TSA review process, but with their approval, any valid visa holder can receive flight instruction in the U.S. and obtain various levels of FAA pilot's license. It's a little unfeasible for vacation visa holders because the review process can take the entire time they're legal to be here, but just because you're in the U.S. for one reason doesn't mean that's the only thing you can legally do while you're here.

Great, thanks lot for informative answer.

@Farhan, Do you know which course that is accepted by TSA? Is it Part 61 and 141 ?

@Vanji I'm not sure, probably both as the end goal is the same, that you'll be a pilot. I couldn't find any information on TSA's website. Maybe you can get it during or after the registration process.

Any information for while you're already in US with F-1 for different program and you're working on STEM OPT? AOPA mentions what if you're in US on F-1 visa and still enrolled for course you can but nothing mentioned for specific scenario that I mentioned above.

You will probably need an M-1 visa for flight training, though you may also be permitted to do flight training on an F series visa (e.g. if it's in conjunction with a college aviation degree program).
This is not really a TSA matter (aside from the parts relevant to the Alien Flight Student Program), but rather a State Department matter. To confirm the specifics and determine which visa type is most appropriate for your planned course of study you should contact the state department (or the US embassy/consulate in your home country).


text:
If I don't want to teach, is there any advantage to getting my CFI/CFII?
The title pretty much says it all. I'm not a good teacher, and I don't have any interest in instructing students. So, if I'm not going to get a job at a flight school, would getting a CFI certificate actually benefit me?
Just for context, I'm a "weekend warrior" that just flies family and friends around because it's fun (although, given just how much fun I'm having, a career change isn't out of the question).

The experience would not NOT benefit you... But what is your long term goal?  It might help as a resume bullet, but then you would have to explain why you have zero instructional hours.  If you are comfortable answering that and have the cash and motivation I say go for it.  You will mostly get opinions here though I think.

Agree that this is pretty close to opinion-based, although there may be some objective answers to be had.  What is your goal: fly to the $200 hamburger on weekends? Job with XXX?  Something else?  Depending on where you want to end up, that may affect the value of the resume bullet (and/or the possible liability of being a CFI but never having instructed).  If you can edit that into your question, that may help you get answers more specific to "advantage" in your particular path.

I put some additional context into the question.

If you have a skill that is lacking (according to you yours is teaching) would it be worth it to try instruction in order to round out your skill sets? Would you be teaching others (even informally) in your commercial aviation pursuits?

The big advantage of getting a CFI/CFII for the instructor is it will make them a better more well-rounded pilot.
The material that is covered in the CFI/CFII course was already taught to you in the instrument and commercial phases of  your training.
The old adage goes... things that are taught may be remembered... if you can teach it to someone else then you have really learned the material. (or something similar to this).
You may be able to fly within standards using your techniques and procedures that work for you.  Those may not work for a different pilot and learning and being able to adapt your techniques and procedures to fit another pilot is eseential from a great instructor.
When you get a job flying SIC in a turbojet, you will learn many new things and their techniques and procedures will be different than yours.  If you have exposed yourself to as many different techniques and procedures before you get that job, the transition will be easier.
Even if you don't teach (I highly recommend it to make you a better pilot), learning the commercial and instrument theory from an instructional teaching standpoint will force you to learn the material better and make you a better pilot.

Some other things Ive seen CFI's do that is not out right teaching flight students:

BFR's. and IPC's Everyone needs em...
My flight club requires a checkout with a club instructor all of whom are members to be able to fly the club planes. I sometimes fly with an instructor just to brush up on things I have not done in a while. While the later is not any "new" instruction it may involve some "teaching"


text:
Why does Vmca decrease with increasing altitude?
A question from a mock MEP class rating test:

Many thanks for your help.

For airplanes with normally aspirated engines or turbocharged engine above their critical altitude, the engines produce less power as altitude increases as atmospheric air density decreases as altitude increases.  As power decreases, the operative engine requires less rudder authority to maintain directional stability due to the reduction in thrust, which allows the airplane to maintain directional authority at lower indicated airspeeds.  Therefore Vmca decreases as altitude increases.

Thanks, would you mind me what a turbocharged engine critical altitude is?

@ob318, it is the altitude above which the engine can't reach the manifold pressure redline. Manifold pressure is limited by the temperature gain from compression, which must remain below autoignition temperature of the mixture, and the strength of the pistons. Since the turbocharger can raise the pressure by some multiple, it could produce much more at low altitudes and the engine must be kept throttled to given MP red-line and thus constant power (power ~ MP × RPM). Above critical altitude the MP decreases even with wide open throttle and with it decreases power.

Wouldn’t rudder control authority also decrease with altitude  as well, for the same reason of reducing air density. Is it that the decrease in (engine) power is much greater in magnitude compared to the decrease in rudder control authority, with increased altitude?

For a given true airspeed, yes.  But Vmca is gauged in indicated airspeed which is proportionate to a constant ratio of dynamic to stagnant air pressures.


text:
Is there empirical evidence that aircraft deliveries increase as air travel increases?

Boeing Co (BA.N) said on Tuesday airplane deliveries rose to 35 in September as it benefited from an uptick in domestic travel
SEATTLE, Oct 12 (Reuters)

An essential principle: greater air travel volume equals greater number of aircraft being manufactured and delivered to airlines.
This Reuters news report seems to allude that is actual number of travel increase which influences the delivery of aircraft, rather than other factors such as predicted future travel numbers, or planned expansion.
Planning air operations and orders surely takes place over a long time scale (months-years). Orders put in as a airline seeks to expand. Orders cancelled with large reductions in demand or business restructuring.
Avoiding ambiguity, I ask: has any empirical data been collected (and be provided in an answer) analysing the relationship between levels of current air travel and levels of aircraft deliveries. How do the timings of travel numbers vs. deliveries compare? I expect there might be lag between these two variables, but how long is this, across the global or various markets?

Well, taking into consideration the timespan of order to delivery with a product like an airliner, it is damn well certain that deliveries do not follow changes in travel in this way under normal circumstances. This is all about covid & and the major FU called MCAS, that caused piling up of acft at various storage locations. As you suspected, normally the causal relation is based on the art and magic of predictions.

Speculation will be required to answer. What Reuters doesn't tell is when these currently delivered aircraft were ordered. Likely a long time ago. There is nothing preventing a customer to cancel or delay an order before delivery, paying predetermined fees, if owning a new aircraft is not economically justified. The increase mentioned may just be the decrease of contract cancellations in reaction to current airline forecast.


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Why is the stab trim set to 4 units nose up during taxi in the Boeing 737, instead of using 5 units universally?

In our airline's SOPs and presumably other airlines too, according to the FCOM, the stabiliser trim is set to 4 units nose up during the taxi-in phase. However, in winter operations, this changes to 5 units to prevent water from snow and ice from running into the tailcone, where it could freeze and affect control surfaces.
I understand the reason for using 5 units in winter, but I'm curious as to why we can't just use 5 units all the time. I haven't been able to find any specific reference to the advantage of using 4 units in the FCOM, FCTM, or AFM.

Is there an engineering or operational reason behind preferring 4 units for normal conditions?
Could it have to do with wind conditions or some other factor during parking?
Is there any known evidence or safety data suggesting that 4 units is better than 5 in non-winter conditions?

It seems like using a single setting year-round would simplify operations, so I'm wondering what makes 4 units preferable in most cases.

Don't know for certain, but a guess is that the stab setting is that which provides a trim speed in the ballpark of the single engine safety speed after rotation, V2+x, and in winter temperatures the performance improvement of the low temp makes that setting tolerable control wise whereas it is not when warm.

That's interesting, I've never heard of guidance to do either of those things.  Running the trim back into the green band, typically around 5-7 units, is quite common, after landing mainly because that's about where it'll need to be for the next takeoff, and if the next crew overlooks enough steps in the checklists to not set the trim to the specific value from the takeoff data, well, at least it's not going to be wildly off when they go to rotate. Our cold weather procedures (which have changed over time) simply require it to be in the green band, not "4.0" or "5.0" or any specific value.

I have found a reference in our OMB that setting stab trim to 4 is a precaution to minimize aft centre of gravity movement (a tipping precaution). Surely it can't have that much impact?


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Why wouldn't the angle markings on an attitude indicator match the horizon line?
This is a video of some guys messing around in a 737 simulator. Starting at about 1:13, when they have the plane in a pretty deep stall, the attitude indicator degree markings no longer line up with the blue-and-brown horizon line. For instance, at 1:15, the markings indicate that the airplane is about 55 degrees below the horizon, but the horizon line itself is only about 20 degrees above the indicator.
So, which one is right? Is the plane 55 degrees or 20 degrees nose-down? And (most importantly) why would the attitude indicator give two conflicting indications like that?
Screenshot from the video at approximately 1:15:

The degree markers are correct, and it's done to improve the readability of the instrument.
If the horizon line was "accurate" at that moment, it would be off the screen and you would be seeing all brown, because the indicator only spans about 45° from edge to edge. Instead of allowing that to happen, the horizon line simply stops moving off the screen when you get much beyond 20° of pitch up or down, so that you always have some blue and some brown to figure out "which way is up". You can see it happening pretty clearly at about 0:34 of the video (when things aren't moving around so chaotically yet). Before that point, the heavy white 0° line coincides with the division between blue and brown, but as the plane continues to pitch up, the 0° line keeps sliding down off the screen, while the blue/brown line stays parked near the bottom of the screen.


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Where is the center of gravity on a tiltrotor?
From what I understand, the CG of full-sized fixed-wing airplanes need to be forward of the wing's center of lift for stability. But the CG of a helicopter must be directly below the rotors since it doesn't have a horizontal stabilizer to counteract the pitching moment.
So, where is the CG in a tiltrotor airplane (the kind that has its engines on the wings, such as the V-22 or the AW-609)?

"From what I understand, the CG of a fixed-wing airplane needs to be forward of the wing's center of lift for stability."-- not accurate-- for example, many free-flight model airplanes have lifting tails, meaning that the CG is behind the center of lift of the wing itself.

@quietflyer I was mostly thinking about full-sized aircraft. I've edited the question to make that more clear.

Looking at photos of the V22 Osprey, it appears that the rotors are pretty much aligned with the quarter chord of the wing, where on most aircraft the CG is located.  So, wouldn't it make sense that the CG is in a small range right there?  How much detail are you looking for?

@MichaelHall I was thinking that the CG would be more forward than that on a fixed-wing.

Why were you thinking that?  (Because it is, after all, a fixed wing...)

If we take the horizontal stabilizer as part of the fixed "wing" arrangement in horizontal flight, then for trim the CG and center of lift must always align exactly (although the rotor/propeller thrust line may affect that slightly). Stability is a bit more complicated and is not relevant to your question.
The trick with a titrotor is to arrange it so that when the rotor tilts up to become a rotary wing, its center of lift still aligns with the CG.
You have to be real careful when loading up a tiltrotor like the V-22, to get the CG right in between the rotor hubs. Cyclic pitch control of the rotor can shift the center of lift a little, but not by very much.

This is pretty vague and generic.  Where is the actual CG?

@MichaelHall I am sorry if "You have to be real careful when loading up a tiltrotor like the V-22, to get the CG right in between the rotor hubs" is not clear. Perhaps you can suggest something clearer?

I didn't say it was unclear, just that it is about as useful as "you have to be real careful to get the CG right in between the fore and aft limits."  But then I didn't like the question either, because it seems intuitively obvious.
(Unless someone is looking for actual numbers.)  But if your answer is accepted I will delete my comments...


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Where can i find aircraft blueprints for study purpose?
I'm a 3D artist and i want to start my new project. I'm going to create an exact model of the Embraer 190 aircraft. I need this for my graduation project at the university (I chose such a project, there is much more than just modeling, but this is the first step). But at the zero step, I was faced with the fact that there are simply no free blueprints in the public domain. There are all sorts of approximate schemes of the aircraft, but they can hardly be called accurate. Even in the manual from Embraer, only exemplary drawings.
Where can I find such things? I am ready to buy them, if necessary.

Not sure about the blueprints, but some 3D models already exist... if it can be of any help: https://3dwarehouse.sketchup.com/search/?q=Embraer%20190

Even more detailed models exist. For example: https://www.turbosquid.com/3d-models/3d-embraer-erj-190-klm/785574
But i still want to create it by myself :)

The downvotes this is getting are unwarranted in my opinion. The question might be a tad naive but it is valid, even if the answer were to be "nowhere".

Fifteen euros gets you what you need here: https://www.the-blueprints.com/vectordrawings/show/1750/embraer_190/
This one is six euros: http://www.lulu.com/shop/elodie-roux/3-view-drawing-plan-3-vues-embraer-190/ebook/product-20233147.html

Thank you, sir! I hope it will help.

High level drawings for familiarization manuals might be what you are looking for, try looking for training material for the ground crew, or familiarization courses for engineers.
Also you lose nothing by reaching out to the manufacturer itself, while it is unlikely you'll get a positive response, it costs little to try.


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What is the difference between iSpec2200 and S1000D specifications?
I know the iSpec2200 and S1000D are the specifications which are used in the aircraft maintenance manual, I have to know how they actually work in the aviation domain and what is the difference between them.

I've only worked with S1000D, but what I understand is that they share a common origin in ATA Spec100, with some differences being:
iSpec2200

maintains a focus on "legacy", monolithic publications
scope is more limited to aircraft
specification costs money (https://publications.airlines.org/CommerceProductDetail.aspx?Product=274)

S1000D

focuses on modular data, with concepts such as data modules and the CSDB
scope is more generalized to any product (air, land, sea, etc.)
specification is free (http://public.s1000d.org/Downloads/Pages/S1000DDownloads.aspx)

Welcome to Av.SE!

iSpec2200 is ATA 2100 + ATA 100 combined and was a means to make these standards more modern with a new improved numbering system and things like gender-neutral terms.  The ATA 100 is from 1956, and I'm not sure about the genesis of the ATA 2100.  Both were retired in 1999.
The S1000D is based on the iSpec2200 so as to enable a simple back-and-forth transition between the two of them.  The main difference in that the S1000D is embracing a digital future for output to web, mobile, pdf, ...  It is based on the authoring of data modules that are small topic-specific modules that are "specification agnostic" and allows the data to be converted to ATA 2100 if that is required by the customer (such as they have a publishing system that takes ATA documents as input) or to any output format imaginable.
And of course the data modules are combined to form a larger publication.

S1000D is the successor of iSpec 2200 since 2012. The standard is now free of charge and can be easily downloaded from S1000D website.

I'm not doubting you, but I am surprised that an organization went from iSpec<something> to just <something> in its naming convention. I mean, dropping the "lowercase 'i', uppercase word" thing in the internet age? Whoda thunk it?? (Unless, of course, Apple sued...) Also, it's odd that they went from the numeric 2200 "backwards" to 1000.  Everything about those two identifiers indicates (to me, at least) that iSpec2200 is newer than S1000D.


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Sudden drop in speed and altitude?
Can someone tell me exactly what happened on this flight? It was a brief moment of sudden drop in altitude (6000 feets) almost at the same time with a drop on engine powers. It's a drop of signal from flightradar trackers or an issue recorded in flight? Thank you!
https://www.flightradar24.com/data/flights/w63655#20da042d

Definitely a glitch. See the FR24 FAQ.

What triggers my question is actually the fact that the "glitch" itself is only for altitude on 13:16 and only after 2 other mins it drops on the speed and then recovers (higher speed than the average before) and then temporary drops again (not at the same rate as initially).

I wouldn't know why a poor signal should be an isolated event.

Smooth descent at 1500 feet per minute from 30,000 to 11,000.  Nothing unusual.
Or was there a blip somewhere in the data that I scrolled past?  (Where?)
Edit: The blip at 13:16 is from 36,000 to 29,600 and back, in less than a minute.  A 12,000 fpm dive followed by a 12,000 fpm climb (and vicious G force at the pull-up) is probably impossible for an A320-232.

Yes there's a blip at/after 13.16 UTC.

Thanks for your input. I'm actually reffering to the mid-phase of the flight, 13:16 where there's a sudden drop in altitude and speed.


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Is there an easy way to determine where a part goes on an airplane?
In order to classify parts for export I need to determine where our parts go on the plane.  (Engine or structural/body)  Specifically looking at the canister, heat shield assy and lock support for the Boeing 767/777.

It might be easier to answer your question if you provided a bit of context.

Already how do you identify these parts. Just numbers or otherwise?

I assume you are not the manufacturer? If so, you need access to some sort of database that will give you that info, In some cases a Google search will answer the question, but in obscure instances you may not find what you want.

Export related questions are best addressed to your compliance department or person. Just saying.

Do you really trust somebody on the internet to classify your part for export for you?

I don't think he's asking anyone on the Internet to do the classification, he's asking if there is a simple mechanism to do that, and besides, if there is, it's probably a web application on the Internet.

J. Hougaard - I wanted to keep my question general on purpose.  user40476 - we have two sources of information to use, but they are not always 100% correct, and I'm trying to verify that my sources/info are/is up to date.  Juan Jimenez - We are the manufacturer in most cases.  I'm just trying to verify our data is good.  AEhere - I am the compliance department - just trying to expand my sources.  vasin1987 - I'm not asking anyone to classify anything.  To All: Thank you for your input and time.  I got the answer I was looking for (thanks Dave).

What you need is a parts catalog/diagram.
Here is one for the 767 that shows parts and where they go. Interestingly all the pictures seem to be of the BBJ (business jet) version of the 767, the other parts should remain the same.


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Are there any regulations on how VFR Aircraft have to dodge if they are facing each other?
I thought about what happens when there are two Aircraft flying VFR, facing each other. VFR Aircraft are not guided by ATC, they have to seperate from other traffic on their own.
I thought it could be quite dangerous if both pilots try to make way and both are flying the same maneuvers.

Is there any regulation on how VFR Aircraft have to fly, to seperate from the other aircraft? Something like: Both should fly to the left or something like that.

Or is this not relevant because this rarely happens and even if, there is enough time to seperate.
I am not asking for any specific country. Tell me what you know.

You asked a question without any country specified, then selected an answer valid only for the US. Fortunately this question is redirected to another post where things are clearer, but you may still improve consistency by adding a tag "FAA" or "USA" (let's imagine Chinese users start to do the same, things will be quite difficult to follow).

Just to educate myself more on StackExchange: Why have I received (so many) down-votes?

Yes, 14 CFR 91.113:

(e)Approaching head-on. When aircraft are approaching each other head-on, or nearly so, each pilot of each aircraft shall alter course to the right.

Here is a good read from AOPA about who has the right of way in a number of different situations, all covered by 91.113.

What if you're inverted?

@Steve Not sure why you would be, but I guess you'd have to go left.

Would that be violating the regulation? (just kidding, of course!)

Not sure if it would be violating that regulation, but if you are performing aerobatics close enough to other traffic that they would have to avoid you, you'd probably be violating some regulation, like careless/reckless operations.

The rule is: both turn (to their own) right.

But the more important rule it the half circle flight level rule.
This says that:

when heading between 000 and 179 degree you fly at an "odd" flight level (IFR, VFR +500ft) like 3500ft, 5500ft, 7500ft,..
when heading 180 to 359 you fly at an "even" flight level (IFR, VFR +500ft) like 4500ft, 6500ft, 8500tf.

This way planes in opposite direction have 1000ft difference in height.

The even/odd altitude rule only applies over 3000 feet AGL (not MSL).

Also, IFR is 1000's, VFR is 500's. So VFR altitudes are 3500, 4500, 5500, etc. IFR altitudes are 3000, 4000, 5000, etc. See 91.179(b) for IFR and 91.159 for VFR.


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What aircraft "don't respond well to tarmac runways"?
On the Shoreham Airfield Website they wrote, they have a tarmac runway for most aircraft and a grass strip (actually two) for "aircraft which need a grass strip".
Quote:

We appreciate that older, more fragile planes don’t respond well to tarmac and that’s why we offer both grass and tarmac runways to cater for a wide range of aircraft.

Page link

What aircraft are those not responding well to tarmac? What does it mean they don't respond well to tarmac? I could imagine this to be old WW2 Aircraft or similar.

My first thought would be a glider aircraft

Some sea planes that need servicing but don't have wheels need to land on grass strips to avoid damaging the pontoons. Here is an example.

@Brilsmurfffje most modern gliders have wheels and could land on concrete/asphalt but it takes some time to taxi them away from the runway... ;o)

I would think that some WW I era biplanes that had a tail skid instead of a tail wheel would be happier landing on grass or dirt. e.g. The Curtis Jenny doesn‘t appear to have a tailwheel but there are pictures of it landing on a paved surface.

@RonBeyer Impressive.

Shoreham is a nice airfield, a very picturesque approach over the coast.

@GdD So true, two hours ago I bought the scenery for flight sim :)

No airplanes absolutely need a grass strip, but for some taildraggers (airplanes which have a tailskid instead of a wheel) it's a real benefit because the grass is softer on the skid, and the friction of the grass improves control on the ground and helps slow the plane down. Pavement isn't that big a problem for skids make from more modern materials, but you can imagine it's not going to do a slab of wood much good. Many vintage designs have no brakes or poor brakes, so having grass makes a big difference when landing, on pavement you'd end up rolling a lot longer and have less directional control at low speed.

Being "kinder" to the skid isn't the only factor: the skid is actually important for steering on the ground, as the rudder doesn't have enough authority at low speeds. On a hard runway where the skid can't dig in, you have little steering control, and a stiff breeze can turn you into wind with no way to recover..


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Are missiles aircraft?
So, at work we have had an ongoing debate for several months now about whether missiles are aircraft or not. We all agree that they're not airplanes of course, but... are they to be considered aircraft? We cannot find consensus, even after long and hefty debate.
The argument for it being an aircraft is that it is a craft (made by humans) that operates in the air.
The arguments for it not being an aircraft are plenty. Some say that they're not registered as such with the authorities, some say in order for it to be an aircraft it needs to be able to transport something or someone.
To end the debate, I just checked the wikipedia page. It's a bit unclear what Wikipedia's stance on the matter is: it mentions that it needs to generate lift from the air for it to be able to be classified an aerodyne, and that missiles don't do a lot of that. But elsewhere it is implies that they do?
I can imagine that this is more of a linguistical question than a technical question, but I still think this site is more suited for it than e.g. ELL-SE. I hope it is allowed here?
I can also imagine this question is very opinionated and thus not suitable to be asked on SE. I am however looking for "opinions" backed up by reasoning and deduction, based on facts. Those facts can be both technical and linguistical in nature.

For what purpose?  We categorize and define things for reasons - safety regulations, airspace use, taxation, etc.  What is the reason or need to either call them aircraft, or not?  Without context it's purely a matter of interpretation and opinion.

Opifex you realize there are two extremely different objects which both have "missile" in the name? https://aviation.stackexchange.com/questions/16215/how-are-cruise-missiles-different-from-ballistic-missiles?rq=1

@Fattie I am aware, yes. However, I disagree that they're "Extremely different" objects. They generally share the same function, and for the average lay person they are pretty much the same thing.
However, if you believe there to be a difference between them in regards to this question: feel free to explain in your answer.

@Opifex - it's hard to see how much clearer the linked question could be eh?  the point of my comment {incredibly, it must be the first time on SE that someone has used comments for the intended purpose :) :) } was to suggest that perhaps the question could be clarified?

Additionally, the missiles "ballistic missiles" that go in to space and back, and clearly incredibly different from the "missiles" hanging under Tom Cruise' fighter plane in movies.

Finally!  "To end the debate, I just checked the wikipedia page". Was that sarcasm???  I just went there and rewrote all the nonsense.

@Fattie what clarification are you expecting in the question? Several good answers have been given already. All of them answer the question perfectly as intended. The distinction between ballistic and cruise is irrelevant for the sake of the question.

If you want to redirect the coworkers, ask what the difference is between bicycle, ebike, and electric motorbike.  Then ask where electric scooters (lime etc) go in the classification.

Cheers @Opifex it's a great QA which I for one certainly voted up. However "distinction between ballistic and cruise is irrelevant..." that seems to be a very dubious thought. The difference between an object which does, and does not, use lifting surfaces, aerodynamic lift, is the difference between, well, a paper plane, and when you make a paper ball to play office-wastebasket-basketball.  The comment just above by Criggie also perfectly presents the issue.

@Fattie It is irrelevant because it is part of the question. For cruise missiles the answer might be "no", for ballistic it might be "yes" or vice versa. It is up to the answerer to make it clear in the answer.
Compare it with "Are bikes motorized vehicles?". The answer could say "Bicycles: yes; motorbikes: no".
Anyways, thanks for the upvote! I'm happy I didn't disappoint you this time either, after your comment on one of my previous question ;) ;)

Let's try to be as objective as possible and using sources to answer this terminology-related question.
ICAO definition of aircraft

Any machine that can derive support in the atmosphere from the
reactions of the air other than the reactions of the air against the
earth's surface.

With this definition, a missile is for sure an aircraft.
FAA definition of aircraft

Device(s) that are used or intended to be used for flight in the air

This, however, depends on the definition of "flight", which is not present in the glossary (it only defines flight time, see discussion here). However, I think we can say that a missile is "able to fly", so I tend to say "yes" also in this case.
EASA definition of aircraft

‘Aircraft’ means a machine that can derive support in the atmosphere
from the reactions of the air other than the reactions of the air
against the earth’s surface.

Same of ICAO, so, yes, missile is an aircraft.

In conclusions, I think that by definitions of all of the major agency a missile has to be considered as an aircraft.

Sources:

https://faa.gov/air_traffic/publications/media/pcg_10-12-17.pdf
https://www.icao.int/dataplus_archive/Documents/20130807/GLOSSARY%20v1%202.pdf
https://www.easa.europa.eu/en/downloads/1720/en

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

I will be accepting this answer since it did exactly what I was hoping for: multiple answers from different perspectives and/or authorities.

Most missiles are aircraft by any sensible definition of the term.

We all agree that they're not airplanes

Au contraire, many missiles are airplanes. Look at a Tomahawk cruise missile- stick a pilot in the front and slap some landing gear on and you basically have a weird-looking airplane. The only thing that truly distinguishes a cruise missile from a typical airplane is the lack of landing gear (something not all planes have anyway- e.g. flying boats).

be an aircraft it needs to be able to transport something or someone

Missiles transport a payload just like any other aircraft. But this is a bad argument anyway- if we rig an airplane to fly without a pilot or payload, it's suddenly not an airplane? Ridiculous.

it mentions that it needs to generate lift from the air for it to be able to be classified an aerodyne

Missiles produce lift just like any other aircraft. How else do you imagine a cruise missile stays aloft?
Many ballistic missiles are spacecraft, not aircraft. By and large, they would work better if the atmosphere weren't there to begin with! There are ballistic missiles that use flight control surfaces to maneuver on reentry, though, and I think it's reasonable to consider these reentry vehicles to be aircraft, specifically gliders.

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

Are missiles aircraft?
Yes, but not for long.

Just like the mushroom that will keep you fed for the rest of your life...

So just curious, would this mean questions about missiles are on topic for this site? (Not directly asking you, OP of this answer, but anyone who knows)

@Wyatt https://aviation.meta.stackexchange.com/questions/4292/should-missiles-be-on-topic

@Wyatt This is Aviation SE, as opposed to Aircraft SE, so any topic about missiles on the premise that that are aircraft might still have an additional semantic hurdle to cross. I think it would be fruitless to seek a simple rule that everyone will agree on, but the more tenuously one's question relates to aviation, the more likely pushback becomes.

I wanted to mark this answer as "Accepted" because I think it's the best one possible, but I think it would give a bad signal to the other people people who have spent a lot of effort in writing their answer :) Upvoted nonetheless!

I agree: it's far too facile to seriously accept. It's less like an answer and more like a pointer to the most profound characteristic of a missile as opposed to other things that fly. -- It would make a better answer to the question "What's the difference between a missile and an aircraft?"


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How is a stator like a diverging passage?
I've read in a lot of places that stators in an axial compressor are responsible for diffusing air and removing the radial velocity to make the flow parallel to the shaft. What I don't understand is how this is accomplished. One way I've heard it explained is this:

With these fundamental points in mind, let us consider the energy transfer between the moving blades and the fluid. For this discussion, consider an axial-flow compressor with such a large ratio of hub diameter to tip diameter that the blades may be approximated by a linear cascade, that is, an infinite row of blades moving in a straight line, as at the top of figure 5.1. If we were to follow an element of fluid as it passed through the moving rotor, we would note changes in both its velocity and its pressure. The character of the changes is more easily seen by transforming to a coordinate system stationary in the rotor, where the flow seems steady, but from an angle as shown by the dashed vectors. If the blades are shaped to turn the flow toward the axis in this coordinate system (in the direction of blade motion), they form diverging passages which (for subsonic flow) result in rising pressure as the fluid passes through the cascade. When this pressure increase is carried back to the stationary coordinate system, as we see from equation 5.5, energy is added to the flow.

Source: Aircraft Engines and Gas Turbines (2nd Edition) by Jack L. Kerrebrock)
Can someone explain in more detail how a stator is like a diverging passage (diffuser?) as Kerrebrock proposes?
An explanation I would propose (but would appreciate confirmation for) is that there is a pressure gradient along the axial flow due to the rotor/stator's generating lift giving them a suction side upstream and a pressure side downstream. This pressure gradient would cause the flow to slow down (and hence increase pressure). Right?

If you take a look at the geometry of the path between two stator blades and compare the cross section of the path(relative to the direction of the traveling gas), you can see, that they form a divergent path.

Thanks for the answer and sorry for the late reply, but I think the logic here is flawed. The green line $A_1$ should be drawn parallel to $A_2$. Then we can make the assumption that all the flow that enters the "diverging nozzle" though $A_1$ and exits through $A_2$. I can't think of any obvious reason why there would be no flow through the dashed green line, which needs to be shown somehow to have a well-defined control volume to apply integral forms of the conservation laws to.

Air needs to move through the engine for it to work.  Turbine engines usually spin above 20k RPM.  If there were not structures to keep the air moving through the motor then it would essentially spin in place and the motor would stall.
If you cut away the side of the engine the compressor rotors would be look similar to \ and the stators would look like /.  The rotor blades and stator blades are at opposing angles so the spin of the air induced by the rotor would be straightened out by the stator blades before hitting the next set of compressor blades.  The goal is to have straight flowing air through the combustion chamber and the reverse is done through the turbine with varying efficiencies depending on how the thrust is used.
If a drive shaft is coming out of the motor the turbine will scavenge as much power as it can to drive the shaft and the compressor leaving very little as thrust.

Thanks for the answer and again sorry for the late reply. It makes sense that the stators will straighten out the air. It even makes sense that the stators will reduce the angular velocity of the air. What doesn't make sense to me is that they will reduce the axial velocity of the air.

I was trying to understand where you are going with this but I just saw your other question about the ideal fluid.  You have piqued my interest as well.


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Can active flight plan be sent for the aircraft in the air?
We are processing some aviation data and I wonder if it is possible that flight plan message can be sent for an active flight (aircraft in the air)?
In the documentation we have the sentence:
"Active flight plan, aircraft is in the air. For ARTS III indicates an overflight"

Your question is short on details, but in general, yes this is possible. A flight could be coming from anywhere, for example:

Another jurisdiction (FIR)
Uncontrolled airspace
A beach somewhere

In these sorts of cases, you may see the first FPL message after the aircraft has departed. The departure aerodrome, if known, may appear in the ADEP field (field 13a), or it may be ZZZZ for an unknown aerodrome, or it may be AFIL for an "air filed" flight plan.

I am trying to understand some flight plans in the data. Could you elaborate on FIR and "Uncontrolled airspace"? In those cases how is FPL message sent and by whom?

FIR = Flight Information Region. Uncontrolled airspace refers to places where there is no ATC. An FPL message may be entered by a controller somewhere when the flight approaches your airspace. It's hard to give you much more detail than that without knowing what data you are processing from whom.


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How is one's hypoxia resistance affected by the time of day?
I am doing my PPL course in the Czech Republic and here they have the following question in tests: when during a day is the resistance of a human organism to hypoxia the best? There are 3 possible answers:

3 am
12 am
5 pm

What is the correct answer and why? Is a human's ability to resist hypoxia connected somehow with the time of day? FAA book doesn't have a word about it.

If it's on the test, it stands to reason that it's in the study material you're expected to use, because that study material would be adapted to the test you'll be taking. What do your Czech books say on the matter?

@CVn couldn't find anything for now. I prefer the FAA book because it's generally much more detailed and better written than the local books.

Hmmm. In my experience, tests aren't designed to facts in general.  They are designed to a specific curriculum, and that will most closely match the books they assigned.   For instance, your course may make certain assumptions, e.g. European VFR operation, which the test questions will also assume.  If you create a more general answer, it may not match the course.  Thus, you can get burned by a "bad" test.

Mildly reduced oxygen saturation can impair night vision without causing other noticeable symptoms. The FAA PHAK mentions this in chapter 17. Maybe this is what the test author was trying to get at with their poorly-worded question.

I have not found a reference to answer this question other than a study of hypoxia resistance in rats at different times of the day. In Russian.

As noted by others here, PPL course questions might not be designed according to pure deductive or inductive reasoning. My approach would have been the following:
Medical-related aviation questions tend to base their background on "old" studies, so let's have a look at Van Liere, E. J. (1964). Resistance to Hypoxia. Archives of Internal Medicine, 113(3), 418.:

As a corollary, it would be anticipated that if the metabolic rate of
  an animal were elevated, the ability to withstand hypoxia would be
  decreased. Zarrowetal in 1951 demonstrated that hyperthyroid states
  decrease tolerance to hypoxia. The thyroid-stimulating hormone (TSH),
  thyroxin, and dinitrophenol all decrease hypoxic resistance.

TSH has a circadian rhythm (24 hour cycle) and levels peak between midnight and 6am (refer to https://medicalsciences.stackexchange.com/a/4034/15791). Therefore, 3am and 12am are not an option, which leaves us with 5pm.
I'm aware that you stated that the correct answer is 3am, and I'll definitely check back with my medical colleagues on that, but the question in itself is clearly not well-defined within the visible context on this platform.
As others mentioned, one measure of gauging the depression of cellular oxidation is metabolic rate, for which in 2014 according to Examining Variations of Resting Metabolic Rate of Adults: A Public Health Perspective no single value for RMR is appropriate for all adults.
Last but not least, if we talk about hypobaric hypoxia, conclusions change again.

It depends upon what you mean by hypoxia resistance.  Keep in mind you may have to take into account hypoxia’s effects on various organs and organ systems of the body and the effects their impairment at different hours.  I’m not going to give away an answer for a test, but that is an additional clue for you to go on.  Good luck.

I've checked with the answers and the correct answer is 3am. Still have no idea why...

Half of the challenge behind piloting exams is understanding what the question is asking. I think this is one example of many poorly-worded questions. AFAIK hypoxia doesn't care what time of day it is, it's all about absorbing oxygen. However, some of the symptoms of hypoxia include reduced colour and night vision, which is bad news for night flight.
If the question doesn't make sense, look at the answers for clues. Out of the 3 options here, 2 of them are very similar - times where it is nighttime year round, and one where it is daylight (varying with latitude and season of course).
So the exam is asking you to choose between day and night, essentially. The symptoms of hypoxia are more abundant at night, so I think you have your answer there.

Psst... above and below certain latitudes, midnight and 3am can be daylight during certain times of the year. ;)


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Are helicopters easier to fly nowadays due to computers?
4 decades ago, someone who seemed to know about helicopters said that hovering in one was like balancing on a ball (like bears do in a circus). But it seems like by using computers hovering and other helicopter control could be more or less automated - is this correct?

How could they not be?

@RobbieGoodwin: I have heard that trains could be completely automated and even airplanes but neither are, at least not for passenger trains/planes. There may be resistance on the part of copter pilots or it may be a harder problem than it appears -- maybe automation works 99.9% of the time but crashes once every 1000 flights. The history of technology is resistance and skepticism. A steam-powered car was created in 1820 or so but horse-drawn carriage drivers fought this and succeeded -- selfish on their part, protecting their jobs.

Yes, a lot of helicopters are easier to fly today due to automation.
Modern (higher-price) helicopters are equipped with autpilot modules which typically range from pilot assistance to full-fledged four-axis autopilot with function from automatically following airspeed, heading and height setpoints over autonomous waypoint flying to features like automatic hover control. A good example is the Helionix suite from Airbus Helicopters which offers exactly these features. Other examples are for example the Bell 505, the Bell 525 (which even features full fly-by-wire) or modern military helicopters like the Eurocopter Tiger, the CH-53K (also fly-by-wire) or the NH-90 (which was the first production helicopter featuring a full-fly-by-wire system).
Perhaps a word on how control is achieved without having a full fly-by-wire system at the example of the Airbus Helicopters (which I happen to know a bit): This control is achieved by inserting actuators with limited bandwidth and control authority (therefore with limited travel) into the control linkage with the addition of a trim actuator. The setup is nicely described here SEMA (smart electro-mechanical actuators). This limited control is then used by the autopilot to provide all its functions. You can also see those Actuators (marked in red as AFCS Actuators) in this picture of the control linkages of a Sikorsky S76:

If the helicopter is flown manually, the Stability augmentation System (SAS) is turned on by standard, which is already a great help. Often you can also choose to select a higher control mode such as the Attitude Command Attitude Hold (ACAH) mode, which lets you directly command the roll/pitch attitude of the helicopter while the autopilot takes care of fine control.
With these systems, helicopters are a lot easier to fly. From personal experience I can tell you that even untrained people have no problem to fly and hover helicopters equipped with such autopilots. However if the autopilot augmentation is switched off, the helicopter is back to "balancing on a ball" just like it was 40 years ago due to the inherent unstable dynamics of helicopters!
So yes, with the right (and expensive) helicopter, it is easier then it was 4 decades ago to fly helicopters.

Expensive?  I can buy a fly-by-wire autopilot for $100 — with the helicopter included!  Of course, it will only be a drone the size of a dinner-plate, but the complexity and materials cost of the autopilot itself goes up only slightly with scale.  Whether the high price-point of fly-by-wire autopilot for full-size helicopter is market failure or compliance issues around manned aircraft I don’t know, but it is not inherent in the device.

@MichaelLorton certainly redundancy and regulation will add a couple zeros. If a drone autopilot fails, it's generally just a warranty or insurance claim.

@MichaelLorton I thinks this stems primarily from the insane cost for certification, coupled with high reliability requirements and low production numbers.
But having designed and worked with UAV and man-carrying systems, I think that the direct comparison is unfair against the man-carrying systems. These are on another level concerning build quality, certification and overall design.

I hope I am not the only one noticing the irony of a safety system being priced out of reach, because of safety considerations.  Admittedly, if an autopilot failed in certain modes, the situation might be worse than no autopilot at all, but still it seems pretty clear that relaxing “safety” processes would save lives.

@MichaelLorton: I love this observation :D. However lower-price helicopters are often operated by amateurs/enthusiasts who really want to fly helicopters by hand for the sake of flying the raw machine. Additionally, these individuals are normally very price sensitive which does not help either

@MichaelLorton your comment was right on the spot. The EASA recently announced to lower the certification threshold for smaller and less expensive helicopters to promote the introduction of autopilots to the market. Autopilots now only have to be certified for a error probability of $10^{-6}$ per flight hour instead of the usual $10^{-9}$ for non-commercially operated helicopters (E.g. Robinson R22 and so on). So I guess, congratulations :D.
On a different note, would you care to "guess" the lottery numbers next week please ;).

My work on helicopter flight controls started more than 30 years ago, and at that time there were already helicopters is service with Automatic Flight Control Systems (AFCS) and Stability Augmentation Systems (SAS). And computers were very expensive at the time.
Fixed wing aeroplanes are designed so that the airframe is inherently stable: when push comes to shove and all systems are down, the pilot is able to maintain control over a stable platform. Exceptions are to be found in the military field, one of the first being the F-16 which had fail-operational inner loop flight control computers (as mentioned in this answer.
Helicopters cannot be designed such that the airframe is always inherently stable - that is, when the rotor is on top.. Helicopters are:

Unstable in the hover, indeed like balancing on top of a large ball. The instability eigenfrequencies have a large enough time constant for humans to learn how to maintain equilibrium.
Subject to cross couplings of the flight controls, which is why larger and more expensive helicopters have mechanical mixers as well.

So automated helicopter SAS has been around for those 4 decades - in more expensive, larger helicopters. Where the SAS is not flight critical and when hovering without it those balancing skills still need to be provided by the pilot.
Complete and always functional SAS requires the system to be fail-operational, by providing redundant system functionality. The F-16 has four identical systems, upon one system fail the other three vote out the failed system, when a second one fails the remaining two can still vote out the failed one, the third failure makes life hard because the airframe is unstable. It is these considerations that make flight critical automated systems expensive, not the cost of the processors and sensors.
Pic from the wiki
Fail-passive SAS in a Robinson R-22 might become a feature in the future, but will the market really require this feature in a low-cost helicopter that is used for teaching unstable hovering skills?

How hard would it be for even an experienced pilot who was hovering when a stability system failed (involuntarily disabled itself) to transition to a stable non-hover flight and then land safely?  If a craft was being used for aerial cinematography, an automated hover system might be needed to accomplish that purpose, and a failure might prevent one from getting the intended shots, but if the pilot could take control of the craft and land it, such failure shouldn't represent a safety risk.

So hypothetically, it should eventually be possible for untrained people to easily take off, fly and land helicopters?

@JonathanReez It seems like the key to many of the new flying machines (not necessarily helicopters) is being simpler to operate than a car. Conventional helicopters wouldn't be able to compete, it seems to me, without becoming way easier to fly.

@supercat yes if the automated system is fail-safe the pilot can take over and land. If in a hover close to the ground tthere is not much leeway and they will need to be alerted. Fail-safety of the system and alerting the pilot would have to be proven effective during certification though.

@JonathanReez yes indeed, starting with single pilot experimental craft which have lower safety requirements (the pilot puts only their own life at stake).

@Koyovis: Would that be an issue at altitudes that were safe for single-engine hovering outside ground effect?

@supercat probably not, unless it would be a hard-over failure or the failure mode would cause an offset to one side.


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Why was the A340 developed with four engines?
The predecessor to the A340 program begun in 1982, when Airbus released the first specification of the designated TA9 and TA11 programmes and in 1986 the TA9/TA11 programmes were officially launched as the A330 and A340 development programmes.
By 1986 Airbus, with their 310 aircrafts, as well as Boeing, with their 757/767 aircrafts, already had twin-jets flying under the ETOPS90 and ETOPS120 rule, introduced in 1985.
Meanwhile Boeing presented the predecessor to the 777 programme, the 767-X ("enlarged" 767) in 1986, but due to customer demand launched the clean sheet 777 twin-jet programme in 1988.
Reading up on that history, I wonder why Airbus did not opt to redesign the A340 with two engines, rather than four.
I judge, that by the mid-80ies, the feasability of two engines for a widebody long-range aircraft with more than 300 passenger was already clear. The loosened ETOPS rules meant, that even regulators were paving the way for such an aircraft.
What was to rationale of Airbus to develop the A340 into a four-engine airliner, while at the same time, Boeing was developing the 777 as a two-engine airliner?
EDIT:
I found another question and answer, which states that ETOPS was the reason. But ETOPS rules were introduced in 1985, while A340 development programme was launched in 1986. So Airbus made a decision despite being ETOPS already in place.

Does this answer your question? Why does the A340 have 4 engines instead of 2?

They did make a two-engine A340.  It's called the A330.

Plus: Boeing had already a big quad engine plane, the 747. Which received an update to the 747-400 around 1989. Airbus followed with the A340. At the same time the also introduced the smaller A330 twin engine, built up on the same design.

Hindsight is 20/20.
At that time, nobody could be sure the ETOPS rule would be extended the way it eventually was. The 1985 extension was a gift from the FAA to Boeing, so US carriers could use the Boeing 767 for transatlantic services. The 1989 extension to 180 minutes allowed them to cover all domestic routes with a single type when before they needed three-engined types (TriStar and DC-10) to fly to Hawaii.
Not everyone at that time was in favor of introducing ETOPS: McDonnel-Douglas said in 1984:

ETOPS "represents a 'totally new risk' that the industry has a limited capability to offset."

and

"… the possible adverse consequences of premature extension of the operating limits can never be balanced by economic gain."

Especially Lufthansa had been sceptical of using two engines for long range and was a driver for the A340 development. Also, Airbus had experienced FAA chicanery before (remember that the A300 was banned for a while because of its slightly higher ground pressure loads). Airbus simply assumed that this kind of chicanery would be repeated when they went with a two-engined long range version.
That only changed when Boeing introduced the 777. Now Boeing was pressing for an ETOPS extension ("early ETOPS") and the FAA duly obliged. But that was in 1995, a full decade later and after Boeing had done extensive ground and in-flight testing, including running engines for a quarter of a million hours. Ironically, the positive experience with the A320 in its first two years of operation helped to convince the sceptics of the merits of early ETOPS.

From the Data I could find, including in the first answer, back at the time the A340 was developed- the standard for ETOPS was 120 minutes, thus making it more profitable to fly a 4 engine aircraft which didn't have to conform with ETOPS rules on shorter routes.
The 777 was introduced a bit later, making it the first airliner to make an entry to Early ETOPS- meaning being able to fly ETOPS 180 right from introduction, in contrary to flying 1 year of safe ETOPS 120 operations (Eventually the JAA didn't agree with this definition and European airliners using the 777 had to prove 1 year of same ETOPS 120 before switching to 180).
My educated guess is that due to development in the field of engine safety Boeing made the bet with the 777, while Airbus didn't. But as you can see- even for them it was not all easy with the certification.